Multiple-stage mineralization in the Sawayaerdun orogenic gold deposit, western Tianshan, Xinjiang: Constraints from paragenesis, EMPA analyses, Re-Os dating of pyrite (arsenopyrite) and U-Pb dating of zircon from the host rocks

- Three generations of auriferous pyrite (Py0, Py1 and Py2) have been recognized.
- U-Pb dating of detrital zircons constrained the maximum deposition age of 355.0 ± 7.3 Ma.
- Auriferous pyrite (arsenopyrite) Re-Os dating yielded the ages of 323.9 ± 4.8 Ma and 282 ± 12 Ma, respectively.
- Py1 is interpreted to have formed in the syn-tectonic stage and Py2 was formed during the late- to post-tectonic stage.
- Os isotopes of auriferous separates from the two stages point to crustal sources of the metals.

The Sawayaerdun deposit, located in the southern Tianshan orogenic belt in Xinjiang, NW China, is one of many orogenic gold deposits in the Central Asian Orogenic Belt (CAOB). The deposit is controlled by reverse faults and hosted in highly deformed carbonaceous turbidites that have undergone pyritic, arsenopyritic, silicification, carbonate and sericitic alteration. Gold occurs mainly in the form of electrum and native gold, and is also present in pyrite and arsenopyrite as “invisible gold”. Three generations of auriferous pyrite have been recognized through detailed petrographic studies and EMPA analyses. The pyrite with framboidal texture (Py0) is disseminated in the host rock, and is locally enriched in gold. The anhedral pyrite (Py1), associated with silicification and quartz veins that experienced strong deformation, has an average Au content of 0.021 wt.%. The euhedral-subhedral pyrite (Py2), showing octahedron cube and pyritohedron habits, is associated with least deformed quartz veins that are best developed in the main orebodies, and has an average Au content of 0.023 wt.%. The Py0 is interpreted to have formed contemporaneously with the ore-bearing rocks, with a maximum depositional age of 355 ± 7.3 Ma based on U-Pb dating of detrital zircons. Re-Os isotopic analyses of four Py1 samples yielded an isochron age of 324 ± 4.8 Ma, and those of five Py2 samples yielded an isochron age of 282 ± 12 Ma. Py1 is interpreted to have formed in the syn-tectonic stage, during the collision between the Tarim craton and the Central Tianshan terrane in Late Carboniferous, whereas Py2 was formed in a later mineralization event, during the late- to post-tectonic stage in Early Permian. These study results suggest that multiple stages of gold mineralization have developed in the Sawayaerdun deposit, and similar mineralization processes may have taken place in other parts of the Western Tianshan Orogen.